Ultrapure water is water very highly purified to have high purity which is obtained by a repeated operation of removing various types of impurities from water containing them. The ultrapure water having very high purity is now specially demanded in production of semiconductor devices which have become very large-scale devices highly miniaturized. A design rule for microfabrication of a semiconductor device has already reached 0.13 μm, and ultrapure water used for the production of semiconductor devices according to this design rule is demanded that the particle diameter of fine particles contained in it is one tenth of 0.13 μm or less. The number of fine particles having a particle diameter of 0.05 μm or more contained in ultrapure water is limited to, for example, 100/L or less.
In order to supply stably ultrapure water meeting the above-described provisions, it is required that a very small number of fine particles contained in the ultrapure water can be counted and monitored at all times.
FIG. 9 is a diagram schematically showing an example of a conventional system for producing ultrapure water. In FIG. 9, a primary pure water treating system 111 produces pure water 112 by removing impurities from raw water such as city water by filtration, cation exchange, deaeration, anion exchange, and passing through an RO membrane. The produced pure water is stored in a secondary pure water tank 113.
A secondary pure water treating system 114 further purifies the pure water 112 to produce ultrapure water. The pure water 112 in the secondary pure water tank 113 is fed by a pump 115 to a UV oxidation treating device 116 where an organic matter is undergone oxidative destruction. The water is ion-exchanged by an ion-exchange tower 117, ultrafiltered by an ultrafilter 118, and supplied as ultrapure water 121 to a use point 120 for various types of usage through a pipe 119 without being stored. Remaining ultrapure water not used is not stored but returned to the secondary pure water tank 113.
For measurement and control of the purity of the ultrapure water produced as described above, it is necessary to measure fine particles contained as impurities in the ultrapure water. A conventional method for measuring fine particles having a particle diameter of 0.1 μm or more contained in the ultrapure water is specified and explained in detail in JIS K0554.
Such a conventional method for measuring fine particles in ultrapure water can be divided broadly into two types. One of them is a method by which a flow of ultrapure water is partly fed to a filtration film through the ultrapure water pipe 119 shown in FIG. 9, the ultrapure water is filtered by the filtration film to capture fine particles contained in it, and the captured fine particles are counted by observing through a microscope such as a scanning electron microscope. The other is a method using automatic fine particle measuring equipment for direct automatic measurement of fine particles in ultrapure water. For example, it feeds a flow of ultrapure water partly from the pure water pipe 119 shown in FIG. 9 to a flow cell, irradiates a laser beam to a flow of ultrapure water in the flow cell and measures the fine particles by using the laser beam scattered by the fine particles in the ultrapure water.
Among them, the method of capturing the fine particles in the ultrapure water by the filtration film and observing and counting the captured fine particles through the electron microscope has been used conventionally as a method capable of measuring the particles with high reliability because it can observe and count the particles closely and can count the particles directly. But, if the fine particles to be measured are very fine and have a particle diameter of, for example, less than 0.05 μm, it is necessary to raise a magnification of the microscope for observing, and time and efforts required for counting increase considerably because the number of fine particles is small.
For example, where ultrapure water containing 100 or less of fine particles having a particle diameter of 0.05 μm or more in one liter is filtered by a filtration film to capture and count the fine particles on the surface of the filter having a diameter of 25 mm and an effective diameter of about 20 mm, it is necessary to flow about one ton of ultrapure water. But, when the filtration film has a smaller pore diameter, a flow rate per unit area of the filter decreases sharply, and filtration time increases considerably. Therefore, if the filtration film has a pore diameter of 0.05 μm, the filtration velocity is slow, and the passage of one ton of ultrapure water takes several months. To monitor the fine particles contained in the ultrapure water and to preserve the quality of the ultrapure water, it is desired that the result is obtained soon, and even when more time is taken to count with high reliability, it is desired that the result is obtained in several days.
Where the fine particles captured by the filter have a smaller particle diameter, a magnification for measuring by observing through the electron microscope is set high, then a field of vision observable at a time becomes narrow, and the number of fields of vision for observing increases. Therefore, lots of time and efforts are required. Besides, the surface of the filter has initial-contaminated fine particles which are called blank particles, the fine particles to be measured have a size falling in a range of particle diameters of the blank particles, and the presence of the blank particles makes it more difficult to measure the fine particles having a small particle diameter in ultrapure water.
The method using the automatic fine particle measuring equipment, namely the method by which a flow of ultrapure water is guided to the flow cell, a laser beam is irradiated to the ultrapure water flowing in the flow cell, and the fine particles in the ultrapure water are measured by using the laser beam scattered by the fine particles in the ultrapure water, has a major advantage that the measured result can be obtained in a short time in comparison with the method of measuring the fine particles in ultrapure water by capturing them by a filter and observing through an electron microscope. Therefore, this method is useful as a measuring method, by which the fine particles in the ultrapure water are always measured and the measured result can be fed back to the control of the ultrapure water.
But, because the intensity of the scattered light from the fine particles is proportional to the 6th power of the particle diameter, the scattered light caused by the fine particles, which is a detection signal, becomes small sharply when the particle diameter becomes small. Meanwhile, output from a light receiver which receives the scattered light involves background noise such as noise from the device itself, scattering caused by the sample ultrapure water itself and the like. Because of the background noise, the minimum particle diameter of the fine particles in the ultrapure water, which can be taken out as a scattered light signal from the fine particles, is limited to about 0.05 μm, and the fine particles in the ultrapure water having a smaller particle diameter cannot be measured.
Japanese Patent Laid-Open Publication No. HEI 3-39635 discloses that scattered light is simultaneously detected by two detectors which are symmetrically disposed with respect to the optical axis, the detected signals are distinguished from background noise to allow measurement of a particle diameter of 0.07 μm or less. But, the ultrapure water has come to have a higher level of purity sharply in these years, so that the particle diameter of fine particles has become much smaller than a range of such particle diameters, and the number of fine particles has become smaller. Therefore, a novel measuring method over the conventional measuring methods is demanded as means for measuring fine particles in ultrapure water.
As described above, the measurement of fine particles which are fine and very few in ultrapure water by the conventional measuring means is quite difficult, and a novel measuring method capable of measuring such fine particles is demanded. The present invention has been made in view of the above circumstances and provides a novel measuring device and method enabling to measure inexpensively fine particles having a particle diameter of less than that of the measured fine particles in ultrapure water.